1. Field of the Invention
The present invention relates to an objective lens system for use within a microscope which is operable in ultraviolet range, in the particular, in far ultraviolet range, in which light has a wavelength of shorter than 300 nm.
2. Description of the Background Art
It is commonly known in the art that a microscope has the property that, assuming that the numerical aperture (NA) of an objective lens system used within microscope remains constant, the shorter a wavelength of light, the better the resolution. Thus, it is possible to observe a sample in greater detail by shortening the wavelength of the illumination light. In addition, illuminating a sample with ultraviolet light would often result in fluorecence of stronger intensity being discharged from a sample than fluorecence obtained by illuminating with visible light. Against this backdrop, a microscope operable in ultraviolet range is preferred in the art, because more information is obtained by observing a sample through such a microscope. Thus, an objective lens system for use within a microscope must be operable in the ultraviolet and/or far ultraviolet range.
Among known conventional objective lens systems which are operable in ultraviolet and/or far ultraviolet range is, for example, an objective lens system for use within a microscope described in "Hikari Gijyutsu Contact, " Volume 25, Number 2, Page 137 (Feb 1987). That objective lens system is illustrated in FIG. 1.
In FIG. 1, an objective lens system 70 includes a first lens 71 made of fluorite, a second lens group 72 and a third lens group 73, disposed in that order from an object side (left-hand side of the figure) to an image formation side (right-hand side of the figure). The second lens group 72 includes two convex lenses 72b and 72c both made of fluorite and a concave lens 72a made of quartz. The second lens group 72 is formed by holding the concave lens 72a between the convex lenses 72b and 72c and joining the same to each other. The third lens group 73 is formed, in a similar manner to the second lens group 72, by holding a concave lens 73a made of quartz between two convex lenses 73b and 73c both made of fluorite and joining the same to each other.
Since the lenses 71, 72a to 72c and 73a to 73c are made of either quartz or fluorite, the objective lens system 70 is capable of transmitting ultraviolet and/or far ultraviolet light, and is hence, operable in the ultraviolet and/or far ultraviolet range.
In addition to this, chromatic aberration can be corrected in the objective lens system 70, since the second lens group 72 is composed of the concave lens 72a made of quartz and the convex lenses 72b and 72c made of fluorite while the third lens group 73 is composed of the concave lens 73a made of quartz and the convex lenses 73b and 73c made of fluorite.
The convex lens 72b, the concave lens 72a and the convex lens 72c of the second lens group 72 are brought into optical contact and joined to each other. Similarly in the third lens group 73, the convex lens 73b, the concave lens 73a and the convex lens 73c are brought into optical contact thereby to be joined to each other. This is attributable to the current technical level which has not as yet been able to provide adhesive which transmits far ultraviolet light. Further, when a junction surface between lenses has to completely eliminate reflection thereat, there is no option other than cementing by optical contact. Thus, in the process of manufacturing the objective lens system 70, junction surfaces must be finished with extremely high accuracy, which results in largely increased costs.
The inventor of the present invention has already suggested an objective lens system for use within a microscope in which such a problem is solved. See Japanese Patent Laid-Open Gazette Nos. 1-319719 and 1-319720. These literature references will be hereinafter referred to as the "prior applications." FIG. 2 shows an objective lens system for use within a microscope, namely, objective lens system 60, according to an embodiment of the prior applications. The suggested objective 60 includes lenses 61 to 63, which are made of either quartz or fluorite. The lenses 61 to 63, i.e., a first to a third lenses, are displaced in that order from an object side (left-hand side in the figure) to an image formation side (right-hand side in the figure) with preselected air spaces therebetween. This enables the objective lens system 60 to be used in both ultraviolet and far ultraviolet ranges. The lenses 61 to 63, as has just been mentioned, are separated from each other; that is, the objective lens system 60 includes no junction surfaces. Thus, because no optical contact is present, the objective lens system 60 is free from the problem relating to manufacture cost.
The objective lens system 60 cooperates with an image formation lens system (detailed structure thereof will be given later) in order to form an image of an object to be observed on the focal plane of the image formation lens system at a predetermined imaging magnification M. In this case, the imaging magnification M is a ratio of the focal length f.sub.2 of the image formation lens system to the focal length f.sub.1 of the objective lens system 60, and is given as: EQU M=-f.sub.2 /f.sub.1 ( 1)
In general, an objective lens system is replaced with another one while an image formation lens system is fixed, thereby the imaging magnification M is changed. Replaceable objective lens systems are therefore necessary, each of the lens systems having a focal length different from the focal length f.sub.1.
The equation (1) shows that a replacement objective lens system which has a focal length of (f.sub.1 /2) is necessary to double the imaging magnification M. If what is required is more than to form the other replaced objective lens system such that its focal length becomes (f.sub.1 /2), the needed objective lens system merely has to have a size of a proportionally reduced objective lens system 60.
However, if the objective lens system 60 is replaced with the needed objective lens system for replacement mentioned above (which is equal to the objective lens system 60 halved in terms of size), the distance between the needed objective lens system and the object to be observed would have to be also halved as long as the pupil of a microscope objective lens is fixed. This is extremely time-consuming as well as labor-consuming in that the microscope must be brought into focus once again all from the beginning after the replacement, and this therefore, enormously adversely affects the operation of the microscope. In addition, such replacement causes halving of the pupil size, which in turn causes a remarkable change in quantity of light illuminating onto the object. On the other hand, if the position of the object is fixed, the objective lens should be moved, so that the position of the pupil would be moved. This should also be avoided in a fixed illumination system for illuminating the object, since positional changes of the pupil exerts unfavorable affect upon the illumination conditions.
Thus, when imaging magnification is to be doubled by replacing objective lens systems, an objective lens system for replacement must have:
(a) a focal length half that of the objective lens system 60; PA1 (b) parfocality; that the, property of eliminating the necessity of bringing a microscope into focus once again after replacement; and PA1 (c) a pupil which has roughly the same size as of the objective lens system 60.